I'm building a 91 GS500E for racing, and part of the rules are that I have to run the stock exhaust.  I'm allowed to update or backdate parts from other years of the GS500E and GS500F though... I see the part numbers did change over the years, I'm curious if there are any appreciable changes that'd justify my tracking down a specific year's exhaust for max stock flow.

As far as I know, all years are the same. It is possible the 01+ may be different since the intake cam changed and then the exhaust cam changed in 04.

This thread below may provide some good info if you need to maintain a stock motor as well.
http://gstwins.com/gsboard/index.php?topic=16773.0

Ah, thanks for the pointer, I knew the carbs changed, didn't realize the cams did as well.

And yeah, I'm not allowed to muck with the motor, other than replacing stock parts with stock parts from other years.  Looks like they all ran the same pistons and heads so no way to bump compression.  I've got an uphill battle against the EX500s it looks like.

The midpipe diameter increased in '04, I believe.  I seem to remember scratch saying that a different year exhaust was too short for his '91 bike...   

yep the exhaust pipes are wider in 04+ .at least the midpipe! not sure about the headers but I can measure them if you want. jut let me know.
I also suggest changing the jets if you haven't done jet. Stock jetting for earlier versions was 122.5 main jet?? or 118?? and stock main jet in 04+ is 130. it should give you some extra power (if you can change the air filter)

Yeah, if you could get some more measurements I'd appreciate it.  No chance the larger external diameter is from Suzuki going to double wall construction to keep the heat off the fairings?

I can change the oem air filter, but I have to stick with OEM style, no pod filters or airbox replacement unfortunately.  Bike will be rejetted on the dyno once I get it running again.  (CDI is toast on one side, tracking down a replacement as we speak.)

Do you know if the 04+ pipes hang any lower, as I don't want to hurt my ground clearance if I can avoid it, it's one of the few areas the GS500 outdoes the EX500?

the midpipe is wider in diameter (When I was making the custom flange for the exhaust I took the info about the diameter from Wiki (it was winter and I didn't feel like going outside and measuring) and I found later that the hole I made was too small. so the diameter is larger.)
I can measure the headers but not today - I don't have calliper in my home. and I am too lazy to do math

I don't think they hang lower (it's just my thought!), but they hang further from the engine to provide extra clearance for oil cooler.

I'll try to measure the headers quickly.
cheers

where do you find completely stock gs500's and ex500's to race?
you start down on power about 15 HP!!!!
but the gs will run the curves much better
small tighter tracks with less straights.....go down 2 t on the CS

the stock suspension is frickin insane to run on the race track

Bought it from a friend actually.  They moved on to a 620 Monster when the GS started having electrical problems.  First a coil died, the replacement was a dual output coil off a 4 cylinder machine, and it eventually arced through the rtv blocking one output, going to the frame and toasting that side of the CDI.  I got it essentially for free with the understanding that it actually gets used rather than taking up space.  New CDI is on it's way.

The EX500s at the track... well lets just say in the 10+ years of this class, I don't think a new EX has been added to the stable, riders come and go, the same bikes just keep changing hands.  They're beat hard, bodged back to life, and beat some more.  I could have gotten into a race prepped EX500, including Penske rear shock, springs and emulators up front, (You're allowed to change shocks and revalve the stock front end) spare wheels, extra sets of race rubber, spare motor, etc for $1200 to $1800.  I didn't feel like picking up something that had a long history of different people hacking it off and on to keep it going.

Loudon is a nice tight technical track, so I'm hoping that it's better ground clearance and handling in general, plus the age and abuse level of the fleet of EX500s I'll be competing against will help make up for the HP difference.  Looking at dynos online, it looks like I'm only down 10hp (20%?!) out of the crate compared to a new EX500, and I think with what I'm allowed to do I can find 3 or 4 hp to bridge that gap some.  Works Perf will be handling the rear, Racetech springs and emulators will be going up front, and I only weight 135lbs, so suspension wise I should be viable.

As far as gearing, I was going to move to a 15 tooth front (Ok, I might drill it for weight) and go to a 45 or so aluminum rear.  I'm down on power already so I don't want to sap it more by going to an overly small front sprocket.

I promised to measure the header and the midpipe diameter for you. Here are the results:

- headers 35mm
- midpipe 55mm

hope it helps.
sorry it took so long but I had really tough week in college.

cheers

Many thanks!  I'll measure my 91 here in a bit so everyone has the data.